The present invention relates to a stereophonic amplifying system constructed of a BTL(Bridged Transformer Less) amplifier including a B-class push-pull output circuit, and more particularly, to a two signal amplifying system such as a stereophonic amplifying system constructed of a one- or two-chip type monolithic IC (semi-conductor integrated circuit).
An ASO (area of safe operation) limiter is typically utilized in the art as the protecting circuit for the output transistor in an amplifying system including a B-class push-pull output circuit. That protecting circuit detects the operating state of the output transistors of the amplifier thereby to render conductive the limiter transistor, which is connected between the input and output of the output transistors, so that the operation of the output transistors may be limited. However, we have found that these output transistors have a drawback in that they oscillate because they have an undesired high-frequency positive feedback loop between their inputs and outputs.
Moreover, we have found that a stereophonic amplifying system including a BTL amplifier has further problems in that the construction of the protecting circuit becomes complex if the aforementioned ASO limiter is provided for all the output transistors (of 4 pieces.times.2) constituting the BTL amplifier. In this regard, an ASO limiter is typically provided for each output transistor in order to prevent the output transistors from being destroyed in case a short-circuiting accident takes place between the two output terminals due to mistaken wiring, insufficient insulation or the like of the speaker connections because the output circuit has an OCL (Output Condenser Less) construction.
On the other hand, by arranging the ASO limiter for those output transistors which are connected between the power source V.sub.CC and the amplifier output OUT at the power source side, which constitues a single power source type B-class push-pull output circuit, those output transistors are rendered nonconductive for any load or ground shorting so that the output transistors at the ground potential side are supplied with no voltage and current and can be indirectly protected. According to our investigations, however, it has been found that the output voltage and current of one channel are fed, when this channel accidentally shorts, through the output terminals to the other channel so that there is a fear of the output transistors of the other channel being destroyed.